<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 100px;
            padding: 20px;
            border: 5px solid black;
            margin: 20px;
        }
        .border-box{
            /* border padding  先分配 剩余的的是内容大小 
            多列式布局的时候有优势*/
            box-sizing: border-box;
            background-color: lightblue;
        }
    </style>
</head>
<body>
    <div class="box border-box"></div>
    <div class="box standard-box"></div>
</body>
</html>